The present invention relates broadly to microwave directional couplers, and more particularly, to a microstrip directional coupler with low coupling coefficients and high directivity.
Microwave couplers of the 20 dB type, for example, with the requirement of high directivity, are commonly used to sample and differentiate between the incident and reflected waves on a microwave transmission line. Couplers of this type, which are implemented with waveguide, coax, or stripline technologies, generally obtain high directivity rather easily because of the media being used. However, a directional coupler implemented with microstrip technology has rather undesirable properties. An example of a conventional microstrip directional coupler is shown in the illustrative embodiment of FIG. 1.
Referring to FIG. 1, a dielectric layer 10 is disposed over the surface of a ground plane 12. On the exposed surface 14 of the dielectric layer 10, there may be disposed a main microstrip transmission line 16 and a coupled microstrip transmission line 18. The two transmission lines 16 and 18 may have a coupling region approximately outlined by the dashed line 20. The coupling coefficient therebetween is generally dependent on the coupling region 20, more particularly, the gap 22 between the transmission lines 16 and 18 and the length 24 of the region 20. The length of the coupling region is generally on the order of one-quarter wavelength of the microwave signal being sampled by the directional coupler.
Because of the electromagnetic properties of the microwave signal conducted over the main transmission line 16, signal components are both capacitively and inductively coupled into the transmission line 18 primarily in the coupling region denoted by 20. In a typical sampling type microwave coupler, the power signal measured at one port of the coupled transmission line 18, like port 26, for example, is generally much smaller than, but representative of, the microwave signal incident to terminal 32 and conducted over the main transmission line 16. Moreover, a power signal measured at the other port of the coupled transmission line 18, like port 28, for example, is representative of the reflected or unwanted microwave signals conducted over the transmission line 16 in the reverse direction to the primary or incident microwave signal flow. A significant power signal measurement at port 28 when the terminal 34 is perfectly terminated is an indication of poor directivity of the microwave coupler.
For the most part, loosely coupled or edge coupled microstrip directional couplers of the type just described generally have directivity which decreases with increasing frequency of the microwave signal conducted over the main line. High directivity, in these cases, becomes even more difficult to obtain as the coupling is loosened. This problem occurs primarily because the propagating velocities of the even and odd modes of the microwave signals conducted over the transmission lines 16 and 18 are not equal.
Various techniques have been used to increase the directivity of the microwave couplers implemented with microstrip technology. For example in one case, capacitive tabs were attached at the ends of the coupling region. However, this technique generally tended to decrease the overall coupler frequency bandwidth. Another technique included the use of overlaying the main and coupled transmission lines in the coupling region with a layer of dielectric therebetween. While this technique provides equal even and odd mode velocities and may be used in broadband applications, there are attendant fabricational problems that frequently make its use undesirable.
Still another technique for increasing the directivity of microstrip microwave couplers is that of wiggling the adjacent edges of the main and coupled transmission lines in the coupling region. The resulting apparatus is commonly referred to as a "wiggly line" microwave coupler. In this technique, the wiggly transmission lines under certain conditions slow the odd mode waves without substantially affecting the even mode waves. However, in a loosely coupled microstrip coupler (say 20 db or greater) with a high dielectric medium such as alumina, for example, and a gap greater than the line width, the wiggling of the transmission lines may effect both even and odd mode waves. Consequently, for this case, any attempt to adequately slow down the odd mode wave by further increasing the severity of the wiggling of the transmission lines is expected to have a similar effect on the even mode wave and thus, be ineffective for the purposes of increasing directivity.
In view of the above remarks, it is quite evident that, to achieve high directivity in a loosely coupled microwave coupler of the microstrip version, something must be done to avoid or alleviate the problems associated with directivity effects arising from the different velocities of the even and odd mode waves between the microwave signals conducted over the main and coupled transmission lines. It is the purpose then of the present invention to resolve this problem area and describe a microstrip embodiment of a loosely coupled microwave coupler with high directivity.